Which impeller for which duty — the short version used by our application engineers.
Backward curved
Blades curve away from the direction of rotation. Highest static efficiency, non-overloading power curve (motor cannot be overloaded when the system resistance drops), stable at high pressure. Runs without a scroll housing (plug fan) directly in an AHU plenum. Choose for AHUs, RTUs, CRAH units, dust collectors and any duty above roughly 300 Pa.
Forward curved
Many short blades curving with the rotation. Moves more air per diameter and speed, so the fan is smaller and quieter for the same airflow at low pressure, but efficiency is lower and power rises steeply if the resistance falls (overloading). Choose for compact equipment: fan coil units, small AHUs, appliances and blowers with scroll housings.
Rule of thumb
Below ~250 Pa and space-limited: forward curved. Above ~300 Pa, variable resistance or energy-critical: backward curved. The selector applies no rule — it checks the measured curves — but the categories on this site are split the same way.